This is a hard, hard question..."should just take her with me"? Only you can know.
I can't give you an answer, but I can tell you this--no matter what you do, you will probably have second thoughts. If you bring your little one with you, it may be that you will have troubles with the flight (ears, new environment), and finding care for her while you do your tests. If you leave her home, you may have to hear about how upset she (and your husband) are.
Ultimately, you have to make the decision of which feels better, and neither may.
I don't know your situation, but as you are going for medical tests, let me ask--do you think you might need some time to process any of this emotionally? If they are routine tests, perhaps not. If they are related to a suspected health issue, maybe so.
For what it's worth, and I say this with all sensitivity, our young children are resilient. They truly are. Unless you absolutely believe that your daughter cannot handle being separated from you for 24 hours, then that's another issue, because you are going to process her being upset differently than a parent who has confidence that their child will be fine. This isn't to say that one way is right, or the other is wrong, but more of a call to listen to your intuition. Children live entirely in the present, so preparation with other caregivers (Grandma and Daddy) may or may not "take", but this is one day in a long life of days to come. Either way.